As was mentioned a little further back, I believe the best case scenario for the US in the coming decades is a Brazil type scenario where communities of right wing whites isolate and wall themselves into segregated communities while left wing whites do the same thing but with (by necessity) even stronger walls in more densely populated parts of the nation. The right wingers will eventually get the idea that racial segregation is the only solution while the left wingers will complain about the right wingers and pretend their actions and even rhetoric aren't effectively identical to them, as long as they get the necessary validation from other left wingers and bots on social media they'll remain completely oblivious and never snap out of it.
A situation like that will become increasingly ungovernable at the federal level as the exodus of productive whites from urban centers lowers the tax revenue and drives them into the ground. We'll see increasingly more examples like Detroit, Chicago and Baltimore emulated across the country, too many for the fed to bail out without tanking the US economy.
Once we reach that point the possibility of mass civil unrest becomes almost inevitable. All that I've seen over the last year or so has convinced me that the left would lose and lose hard if it came down to a contest of violence. Our current circumstances are nothing like the Union vs the Confederates, there's no big contiguous areas of roughly equal size belonging to each side. If mass bloodshed occurs anywhere the nation as a whole will be primed for it and fighting will quickly spread and across the nation, battles will happen county to county, city to city, sometimes block by block and military base to military base.
The nature of US geography and how it correlates to demographics greatly favors the right wing in that situation. The vast majority of life's necessities are produced and travel enormous distances through areas they control and all the wealth of big metro areas that make up most of the lefts power base is generated by selling products and services to the people their at war with. Land locked places like Minneapolis and Austin would be fucked in the first days of the war and coastal cities wouldn't be far behind them.